The genus name Procompsognathus, means "before elegant jaw", and is derived from the name of another dinosaur, Compsognathus. A later (Jurassic) small predatory dinosaur, Compsognathus means "elegant jaw", and is derived from the Greek word kompsos (κομψός) which can be rendered as "elegant", "refined" or "dainty" and the Greek word gnathos (γνάθος) which means "jaw". The prefix Pro (προ) implies "before" or "ancestor of", although this direct lineage was not supported by subsequent research. The specific nametriassicus refers to the geologic time period to which this dinosaur belongs, the Triassic.[1]

Procompsognathus may have been about one meter long (3.3 ft),[2] though Fraas in 1913 estimated a length of 75 cm (2.5 ft). In 2010 Gregory S. Paul gave an estimate of one kilogramme for the weight.[3] A biped, Procompsognathus had long hind legs, short arms, large clawed hands, a long slender snout with many small teeth, and a stiff tail. The femur discovered for the type specimen of this genus measures 93mm and the tibia, 112mm. The tibia is approximately 20% longer than the femur in Procompsognathus, an adaptation which has been strongly correlated with the development of cursorial habits in dinosaurs, suggesting that they were good runners.[4]

The fragmentary and poorly preserved skeleton of Procompsognathus was found in the Middle Stubensandstein member of the Löwenstein Formation at the Weiße Steinbruch, the quarry of Albert Burrer on the northern slopes of the Stromberg region near Pfaffenheim in Württemberg, Germany.[5] The discovery was made by Albert Burrer in the spring of 1909 in white sandstone and gray/blue marl sediments that were deposited during the Norianstage of the Triassic period, approximately 210 million years ago.

The holotypeSMNS 12591, consisted of three blocks of sandstone: one showed a small, seven-centimetre-long, severely crushed skull with lower jaws. The second and third contained the partly articulated remains of a postcranial skeleton, including twenty-nine vertebrae of the neck, back and tail; ribs; elements of the shoulder girdle and a forelimb; an ilium; both pubes and the hindlimbs. It represents an adult individual.

Burrer sent the specimen to Professor Eberhard Fraas of the königliche Stuttgarter Naturalienkabinett. In a lecture on 9 October 1911, Fraas referred to it by the name "Hallopus celerrimus", considering it jumping form of dinosaur that was approximately 60 cm (2.0 ft) long, and was associated with the origin of birds.[6] Later, Fraas decided to use a different name in the official publication.[7] Fraas in 1913 named the genus Procompsognathus with as type species, Procompsognathus triassicus.[8] The type specimen is housed in the collection of the State Museum of Natural History in Stuttgart, Germany.

In 1921, Friedrich von Huene referred two more specimens, both also found in the Burrer quarry, in 1908, to Procompsognathus: SMNS 12352, a partial skull and lower jaws from a larger individual than the holotype, and SMNS 12352a, an isolated left hand.[9]

While it is undoubtedly a small, bipedal carnivore, the extremely poor preservation of the only known Procompsognathus fossil makes its exact identity difficult to determine. Fraas originally assigned it to the Dinosauria. In 1923 Franz Nopcsa coined a Procompsognathinae, and in 1929 von Huene created a Procompsognathidae, though these concepts are today rarely used. In 1932 von Huene saw it as a member of the non-dinosaurian Pseudosuchia.[12] It has since then usually been considered a theropod dinosaur, with some exceptions. In 1992 Paul Sereno and Rupert Wild stated that the holotype specimen consisted of fossils from two separate animals: the postcranial skeleton would be a theropod, likely a coelophysoid related to Segisaurus, but the skull and the von Huene specimens they referred to the basal crocodylomorphSaltoposuchus connectens.[13] However, in 1993 Sankar Chatterjee after further preparation refuted their assessment and regarded the skull as that of a theropod similar to Syntarsus, and demonstrated that it could not have been a crocodylomorph, as it lacks the landmark features of this group.[14][15] Sereno (1997) and Ezcurra and Novas (2007) conducted phylogenetic analysis that supported the placement of Procompsognathus in the taxon Coelophysidae.[16][17] This genus may be most closely related to Segisaurus halli.[18]

Much controversy has arisen however, about the association with the later material referred by von Huene. In 1982 John Ostrom claimed that SMNS 12352 and SMNS 12352a originated from a taxon different from the holotype. In 2006 and 2008 Fabien Knoll concluded that SMNS 12352 represented a crocodylomorph and SMNS 12352a a crocodylomorph or some other basal archosaur. The postcranial skeleton, for which she reserved the inventory number SMNS 12591, was a coelophysoid; and the skull, now indicated with the number SMNS 12591a, a perhaps more derived theropod, possibly a basal member of the Tetanurae.[19][20] In 2012 Knoll after a CAT-scan reaffirmed that SMNS 12352 was a crocodylomorph, but established it was different from Saltoposuchus.[21]

Oliver Rauhut and Axel Hungerbuhler (2000) noted features of the vertebrae which suggest that Procompsognathus may be a coelophysid or ceratosaur,[22] and Carrano et al. (2005), in their re-study of the related genus Segisaurus, found both Segisaurus and Procompsognathus to belong to the Coelophysidae within Dinosauria.[23] In 2004 David Allen considered Procompsognathus to be a primitive, non-dinosaurian ornithodiran.[24]

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group. In 2000, Rauhut noted that its scapula is more slender than that of Coelophysis bauri.

In 1998 Chatterjee noted that the skull of Procompsognathus possesses a suite of theropod synapomorphies, which included:

^Knoll, F. & Schoch, R., 2012, "CT scanning, rapid prototyping and re-examination of a partial skull of a basal crocodylomorph from the Late Triassic of Germany", Swiss Journal of Geosciences105: 109-115